1. Field of the Invention
The invention relates to a process for the cost-optimized preparation of organopolysiloxane resins by the hydrolysis and condensation of silanes and/or their partial hydrolyzates, and to their use.
2. Background Art
Processes for the preparation of organopolysiloxane resins are already known. DE-A 41 32 697 describes a process for the preparation of organopolysiloxane resins by the acid hydrolysis of alkoxysilanes or their partial hydrolyzates, followed by a basic aftertreatment and subsequent precipitation in water. DE-A 42 16 139 describes a process for the preparation of organopolysiloxane resins by the hydrolysis of alkyl silicate in the presence of acid, wherein part of the alcohol formed is distilled off prior to basic reaction in a water-insoluble organic solvent, and wherein, after neutralization, the precipitated salt is removed and the reaction mixture is optionally distilled. DE-A 42 43 895 describes a process for the preparation of organopolysiloxane resins by the hydrolysis of alkoxysilanes and/or their partial hydrolyzates in the presence of polybasic acids, wherein, after partial neutralization of the acid, the alcohol formed is partially distilled off before a subsequent basic reaction in a water-insoluble organic solvent, and wherein, after subsequent buffering to a pH of between 6 and 8, water or alcohol is distilled off again and precipitated salt is filtered off.
The foregoing processes have the disadvantage that it is usually very difficult to filter off all of the salt precipitated in the neutralization. A first filtration frequently leaves a slight turbidity, giving the product a bluish opalescence that is unacceptable for quality reasons. This can generally be removed only by several filtrations. Particularly in the preparation of products of higher viscosity (>100 mPa·s), filter performance decreases dramatically with only a slight increase in viscosity and it becomes impossible to remove the opalescence within economic periods of time.
For these reasons, the salt is often filtered off earlier, at the stage of a low-viscosity intermediate. Only then is the higher-viscosity target product prepared by subsequent process steps, e.g. distillation. On the one hand, this procedure demands a greater effort and is thus more cost-intensive, and on the other hand, a turbidity can reoccur during the distillation to give the high-viscosity end product, which makes the preparation process very time-consuming and hence also cost-intensive.